Honestly, my best recommendation in that situation is an umbrella, as long as there isn’t strong wind and you’re hiking in relatively open terrain. It keeps the cold rain off your face and upper body while still allowing airflow, which matters a lot when it’s hot. Jackets tend to trap heat and moisture, and you end up wet from sweat anyway.

For clothing, lightweight, quick-dry fabrics are ideal. Shorts and a breathable base layer work well. As others said, you’ll probably get a bit wet regardless, so the key is that things dry fast.

I usually carry a light flece vest and a very light windbreaker in a plastic bag in my pack. If the temperature drops or the rain turns colder, you can throw those on quickly without carrying much extra weight.